This invention relates in general to high power, recirculating gas lasers.
Lasers of all types generally operate with rather low efficiency. Even the carbon dioxide (CO.sub.2) laser, which is one of the most efficient, requires ten or more times greater input power than it provides output laser power. All of the waste heat generated by this inefficiency must be removed from the laser while maintaining the laser medium at a moderate temperature which is generally required for successful operation.
One satisfactory technque for achieving high power continuous gas laser operation is the use of a gas recirculation and cooling system. A glow discharge for pumping the laser is maintained in a plasma excitation region, from which the laser energy is extracted by a resonant optical cavity. The recirculation of the gas flow directs the hot gases through a heat exchanger where the gases are cooled, and the heat extracted is rejected to the outside atmosphere. From this heat exchanger the gas then flows through a blower which recirculates it back to the plasma excitation region, and so on.
The resonant optical cavity for a gas laser is formed by mirrors mounted on the ends of a structure in such an arrangement that light photons can be stored between the mirrors and can interact with the excited laser plasma. The output beam is typically obtained from a partially transmitting mirror in a stable resonator configuration, or from the light passing around the mirror in an unstable resonator configuration. In either case the mirrors must be maintained in extremely precise relative angular orientation and spacing to maintain a resonant cavity such as is necessary for constant laser output and for beam geometrical shape or mode quality.
The optical resonator structure for a high power laser must perform the extremely difficult task of maintaining the precise angular orientation of the mirrors while being situated very close to the plasma excitation region from which large amounts of heat are flowing. Because of thermal expansion, any non-uniform temperature change in the resonator, even over very long time periods, will change the laser output power and mode quality. Most high power gas lasers have experienced these problems because the extremely hot gases downstream of the plasma excitation region have not been sufficiently isolated from the resonator structure. Thus, these hot gases flowing over the resonator mirror supports have resulted in non-uniform thermal expansions in the resonant optical cavity, particularly in the mirror support structures.